Establishment of polarity in the egg can be described as the earliest step in embryonic patterning. A single cell, the fertilized egg, contains the information that will ultimately specify the entire organism. As the egg divides during the early stages of development, different cells acquire different fates. One mechanism to account for this difference in developmental potential is the sequestering of molecules in the egg that could specify such potential. It is a long-standing proposal that localization of maternal factors in eggs can provide the basis for pattern in the early embryo, and while there is evidence for the existence of localized cytoplasmic determinants in many systems, in no case is the localization process understood. This research proposal addresses this problem by investigating the localization of specific maternal mRNAs in the frog egg. The aim of the proposed research is to understand the means by which RNA is localized to specified regions within a cell. The Xenopus oocyte provides an excellent model system in which to study this problem. Chief among the experimental advantages in this system are a rapid injection assay for localization, and ease in obtaining large amounts of material for biochemical studies. Three lines of investigation are proposed to study RNA localization: (1) Experiments are proposed to identify and isolate components of the localization apparatus. The biochemistry of RNA localization is not currently understood. By addressing this issue, new information should be gained concerning the molecular apparatus involved in moving macromolecules to defined regions of the oocyte cytoplasm. (2) To address certain mechanistic questions, experiments are proposed to probe the role of the cytoskeleton in RNA localization. (3) In order to allow understanding of how particular RNAs are recognized as requiring localization, experiments are proposed to characterize the RNA signal for localization. These experiments may provide insights not only into how developmental signals are localized, but also the role this may play in cell type determination. Currently, it is not understood how developmental signals are localized to determine cell type during early development. The experiments proposed here address that problem, and may contribute towards understanding how developmental programs are disrupted in certain diseases. To understand the means by which cellular processes can go awry, it will first be necessary to gain a basic understanding of normal development and differentiation.